Both corner cube retro-reflectors and spherical retro-reflectors are optical designs that redirect an incoming beam of incident light from a wide range of viewable angles back towards the source of the incident light without the need for moving optics or other assemblies.
Corner cube retro-reflectors make use of angled reflecting surfaces to reflect incident light back to the source. Corner cube retro-reflectors can have exposed protruding reflective surfaces, such as polished metal or other coated surfaces, or can be internally reflective, for example lenses cut to have an internal corner at one end and a face to receive light at the other end. Incident light that enters the corner-cube is reflected between the surfaces such that it exits in the same direction as the light entered.
Spherical retro-reflectors utilize beads that are embedded in a reflective material, such that incident light enters the front of the bead, is refracted as it enters the bead, is reflected by the reflective material at the back of the bead, and is again refracted by an equal amount as it exits the bead. In this manner, the incident light is returned in the same directions as the light entered.
U.S. Pat. No. 6,045,230 to Dreyer et al. describes a Modulating Retroreflective Article having a back surface and a front surface separated by a distance. The back surface has alternating retro-reflective areas and separation areas and the front surface has alternating areas of different transmission characteristics. The Modulating Retroreflective Article modulates the incident light based on the relative displacement of the back and front surfaces and the angle of the incident light. Physical movement of the article or changes in the angle of incident light modulates the light and therefore can be used to transmit information. However the system is not adapted to a communications system. Moreover, a system that requires physical movement limits the data rate at which information can be transmitted, both because physical systems have limits as to how quickly they can be moved to cause the modulation, and because there is a limit as to how accurately the physical changes can be transformed into detectable modulations. Moreover, systems that rely upon physically moving elements are susceptible to system noise such as vibration and other perturbations of physical systems. The use of a physically spaced plate in front of the retroreflector also reduces viewable area of the reflector component, while imposing a reduced effective operating angle for the entire assembly as light will be blocked or interfered with by the front mask as incident angle increases.